net/http: net/http: doc that prefer "must" over "should"

See https://go-review.googlesource.com/c/go/+/59850

Change-Id: I9f0b6bc009eae86cbbdb56562ee4eb8d5eef653e
Reviewed-on: https://go-review.googlesource.com/61230Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/server.go

@@ -179,7 +179,7 @@ type Hijacker interface {
 	// The returned bufio.Reader may contain unprocessed buffered
 	// data from the client.
 	//
-	// After a call to Hijack, the original Request.Body should
+	// After a call to Hijack, the original Request.Body must
 	// not be used.
 	Hijack() (net.Conn, *bufio.ReadWriter, error)
 }